Optical pickup device and optical disc driving apparatus having light polarizing hologram

ABSTRACT

An outgoing light beam from a laser light source 42 of a laser module 41 is radiated via a light polarizing hologram 51 on a two-segment optical rotation plate 52 of a movable unit. The two-segment optical rotation plate 52 is comprised of a right rotation plate 52R and a left rotation plate 52L divided along a line extending along the radius of an optical disc with the optical axis as the center, and rotates the outgoing light from the laser light source 42 by pre-set angles. The two-segment rotation plate 52 is arranged between the light polarizing hologram 51 and an objective lens 53. The outgoing light, rotated by the two-segment rotation plate 52, is radiated on the optical disc 33 via the objective lens 53. The reflected light from the light polarizing hologram 51 is passed via the objective lens 53 to the two-segment rotation plate 52 so as to be rotated further by pre-set angles by the right rotation plate 52R and the left rotation plate 52L. The light polarizing hologram 51 bends the light path of the outgoing light from the optical disc 33, rotated by the two-segment rotation plate 52, responsive to the direction of light polarization, and radiates the outgoing light. The focusing error signals (FE signals) or magneto-optical signals (MO signals) are derived on the basis of detection signals by photodetectors 44 to 48 receiving the reflected light from the optical disc 33 incident via the light polarizing hologram 51.

TECHNICAL FIELD

This invention relates to an optical pickup device and an optical discdriving apparatus adapted for employing a variety of optical discs, suchas a play-only optical disc, a recordable phase-change optical disc or amagneto-optical disc.

BACKGROUND ART

As optical discs for recording/reproducing the information byirradiation of a laser beam, a magneto-optical disc, a variety ofwrite-once optical discs, a digital audio discs, such as a so-calledcompact disc or an optical video disc, have been put to practical use.

Of these optical discs, the compact disc or the optical video disc is aplay-only disc, and is comprised of a transparent substrate, in whichdata pits corresponding to information signals are formed as recesses,and a reflective layer formed on the transparent substrate.

As the transparent substrate, a disc-shaped substrate, formed byinjection molding a resin, such as polycarbonate, is predominantlyemployed, since the cost may be lowered especially in case of massproduction. On concentric or spirally extending track(s) of thetransparent substrate, there are formed data pits as recesses. Thereflective layer is layered on the surface of the transparent substratecarrying the data pits. In general, an Al reflective film is usedbecause it has high reflectance and good thermal conductivity.

With the above-described play-only optical disc, the difference in theamount of reflected light between the pits and mirrors, that is discportions devoid of the pits, on laser light irradiation from thetransparent substrate, is detected, and a bit pattern on the track(s) isaccordingly reproduced.

For correctly reproducing error-free signals by the above technique,laser light spots need to be radiated correctly on a track on which abit pattern to be read out is formed. To this end, the optical discdriving apparatus performs tracking servo of the optical pickup device.Among optical disc systems for scanning the concentric or spirallyextending track(s) with the laser light beam for recording/reproducingvarious sorts of data, there are known a CLV system for rotationallydriving the optical disc at a constant linear velocity (CLV) forrecording/reproducing data, and a CAV system for rotationally drivingthe optical disc at a constant angular velocity (CAV) forrecording/reproducing data. There are also known a continuous servosystem in which tracking control is done using a continuous pre-grooveformed along the track, and a sample-servo system in which trackingcontrol is done using servo areas provided discretely on the track(s).

A conventional optical pickup device for a magneto-optical disc isconfigured as shown for example in FIG. 1. During reproduction orrecording, a laser beam as an outgoing light beam of a P-polarizedcomponent is radiated from a laser diode 1. This outgoing light iscollimated by a collimator lens 2 so as to be shaped by a shaping prism3 to fall on an S-polarizing beam splitter 4.

The S-polarizing beam splitter 4 has a light polarization beam splitterfilm 4a having characteristics of reflecting the S-polarized lightcomponent having the direction of polarization perpendicular to theP-polarized light component and reflecting and transmitting 50% of theP-polarized light and the remaining 50% thereof, respectively. Thus,one-half of the outgoing light of the P-polarized light, incident on theS-polarizing light beam splitter 4, is reflected, while the remainingone-half thereof is transmitted. The outgoing light transmitted throughthe S-polarizing beam splitter 4 is reflected by a 45°-mirror 5 andthence radiated via an objective lens 6 on a magneto-optical disc 7.

During recording, data is supplied via an input terminal 8 on a magnetichead 9. This drives the magnetic head 9 responsive to the datafor.generating a magnetic field corresponding to the data. This magneticfield is impressed on an area of the magneto-optical disc 7 irradiatedwith the laser beam for recording data thereon.

A reflected light beam is produced by the magneto-optical disc 7 beingirradiated with the outgoing light. This reflected light is reflected bythe 45° mirror 5 via the objective lens 6 to fall on the S-polarizingbeam splitter 4.

This reflected light is polarized responsive to data recorded on themagneto-optical disc 7 and thereby reflected as an S-polarized lightcomponent. The amount of the polarized light is delicate and the majorportion of the reflected light is the P-polarized light component. TheS-polarizing beam splitter 4 reflects 100% of the S-polarized lightcomponent, while reflecting and transmitting 50% and the remaining 50%of the P-polarized light component, respectively. Thus, as for thereflected light, the reflected portion of the S-polarized light beam isreflected in its entirety by the S-polarization beam splitter 4 to fallon a polarizing beam splitter 10, while one-half and the remaining halfof the P-polarized light component are reflected by and transmittedthrough the S-polarizing beam splitter 4, respectively.

The polarizing light beam splitter 10 has a polarization light beamsplitter film 10a has characteristics of transmitting the P-polarizedlight component in its entirety and reflecting the S-polarized lightcomponent in its entirety. Consequently, as for the reflected lightincident on the polarizing beam splitter 10, the reflected light of theP-polarized component is transmitted through the polarization light beamsplitter film 10a to fall on a servo signal detection system 11, whilethe light of the S-polarized component is reflected by the polarizingbeam splitter film 10a so as to fall on a data detection system 12.

The reflected light of the P-polarized light component, incident on theservo signal detection system 11, is converged by a lens 13 and acylindrical lens 14 to fall on a photodetector 15 used for detectingservo signals. The photodetector 15 receives the reflected light of theP-polarized light component and supplies a detection signalcorresponding to the received light to a servo signal generatingcircuit, not shown. The servo signal generating circuit generatesfocusing error signals and tracking error signals, based upon thedetection signal from the photodetector 15, and transmits the focusingerror signal and the tracking error signal to servo control circuits,not shown. These servo control circuits effectuate tracking errorcontrol and focusing error control based upon the focusing error andtracking error signals. This assures data reproduction under just-trackand just-focus conditions at all times. The S-polarization light beamsplitter 4 reflects 50% of the P-polarized light component, whiletransmitting the remaining 50% thereof, while the servo signal detectionsystem 11 detects the tracking error. and focusing error signals basedupon the reflected light of the P-polarized light components reflectedby the S-polarizing light beam splitter 4. Since the major portion ofthe reflected light is the P-polarized light component, the tracking andfocusing errors may be detected with a sufficient light volume if theS-polarization light beam splitter 4 is configured for reflecting andtransmitting 50% and the remaining 50% of the P-polarized lightcomponent, respectively.

The reflected light of the S-polarized light component, reflected by thepolarization beam splitter 10, is converted by a λ/2 plate 16 of thedata detection system 12 into a reflected light of the P-polarized lightcomponent which is then incident via a condensing lens 17 on apolarization beam splitter 18. The polarization beam splitter 18 has apolarization light beam splitter film 18a having characteristics ofreflecting 50% of the P-polarized light component and transmitting theremaining 50% thereof. Thus the reflected light of the p-polarized lightcomponent, incident on the polarization beam splitter 18, is divided bythe polarization beam splitter film 18a into two portions which areincident on data-detection photodiodes 19A, 19B.

The photodetectors 19A, 19B receive the two reflected light beams andtransmit detection signals of signal levels corresponding to the volumesof the received light to a data detection circuit, not shown. The datadetection circuit detects data based upon.the detection signals andtransmits the detected data to a data processing system. The dataprocessing system processes the data in a preset manner and transmitsthe processed data to an external equipment, such as a computer or aspeaker.

There has also been known a phase-change type optical disc 24 forrecording data by exploiting changes in structure between the amorphoussate and the crystal state of a substance.

The optical pickup device, reproducing data from the phase-changeoptical disc, has a structure as shown in FIG. 2, and is configured forradiating a laser beams as an outgoing light of, for example, theP-polarized light component. The outgoing light is collimated by acollimator lens 21 and reflected by a 45° mirror 22 to fall on ahologram film 17.

The hologram film 27 is formed as a planar hologram in the shape of arefractive lattice functioning as a polarization beam splitter fortransmitting the light of the P-polarized light components as it is andfor radiating the light of the S-polarized light component afterchanging its light path. Thus the outgoing light of the P-polarizedlight component, incident on the hologram film 27, is directlytransmitted through the hologram film 27 to fall on a quarter wave plate28. The quarter wave plate 28 converts the linear-polarized radiationinto circular polarized light which is radiated via an objective lens 29on the phase-change optical disc 24.

The circular-polarized outgoing light is radiated on and reflected bythe phase change optical disc 24, whereby the circular-polarizedreflected light, opposite in the direction of polarization to theoutgoing light, is produced. This S-polarized reflected light falls viathe objective lens 29 on the quarter wave plate 28. When thecircular-polarized light, opposite in the direction of polarization tothe circular-polarized laser light beam, is incident on the quarter waveplate 28, the quarter wave plate 28 converts it into a reflected lightof the S-polarized light component. This reflected light of theS-polarized light component falls on the hologram film 27.

The hologram film 27 has characteristics of functioning as a polarizingbeam splitter for bending the light path of an incident light of theS-polarized light component a pre-set angle and radiating the lightalong the bent optical path. Thus the reflected light of the S-polarizedlight component, incident on the hologram film 27, has its light pathbent by a pre-set angle by the hologram film 27, so as to be radiated ontwo photodetectors 26a, 26b of the laser module 20 via a 45° mirror 22and the collimator lens 21.

The photodetectors 26a, 26b receive the reflected light and outputdetection signals corresponding to the received light volumes. Thesedetection signals are supplied to signal processing systems, not shown.These signal processing systems detect the focusing and tracking errorsignals and data recorded on the phase change optical disc 24, basedupon detection signals from the photodetectors 26a and 26b, and transmitthese to a servo control system and to a data processing system. Thisenables the data to be read under the just-track and just-focus states.

The optical pickup device for the phase-change optical disc, shown inFIG. 2, is configured for bending the light path of the reflected lightby exploiting the characteristics of the hologram film 27. Thus thereflected light can be received by the photodetectors 26a, 26b providedin the vicinity of the laser diode 25; thus enabling the overall lightpath to be reduced. In addition, the optical pickup device itself andthe apparatus provided with such optical pickup device can be reduced insize.

However, with the optical pickup device for the magneto-optical disc, inwhich the outgoing light and the reflected light are split using thepolarizing light beam splitters 4, 10 and 18 for detecting the data andthe focusing and tracking error signals, the number of component partsand hence the costs are increased. In addition, since it is necessary tosecure light paths for the reflected lights split by the beam splitters4, 10 and 18, the optical pickup device itself is increased in size.

On the other hand, the P-polarized light components are reflected andtransmitted in amounts of 50% by the polarizing light beam splitter 10.This reflectance is set on the basis of the volume of light radiated onthe servo signal detection system 11 and the shot noise of thephotodetector 15 for servo signal detection or the noise produced bydouble refraction by the magneto-optical disc 7. The coupling efficiencyand the S/N ratio are related to each other by trade-off, such that, ifthe coupling efficiency is increased, the S/N ratio is lowered, whereas,if the S/N ratio is improved, the coupling efficiency is deteriorated.

Although the light path length may be reduced with the optical pickupdevice for the phase change optical disc for reducing the size of theoptical pickup device itself, it is difficult to detect the playbackdata of low signal intensity from the magneto-optical disc because thequarter wave plate 28 is employed.

It is therefore an object of the present invention to provide an opticalpickup device and an optical disc driving apparatus whereby thepolarizing beam splitter is eliminated and the number of components maybe diminished for reducing the size and cost.

It is another object of the present invention to provide an opticalpickup device and an optical disc driving apparatus whereby the couplingefficiency and the S/N ratio may be improved for satisfactorilyreproducing data recorded on the magneto-optical disc.

DISCLOSURE OF THE INVENTION

An optical pickup device according to the present invention includes alaser light source for radiating a laser light, an objective lens forradiating an outgoing light from the laser light source to an opticaldisc, optical rotation means arranged between the laser light source andthe objective lens and being split into a right rotation plate and aleft rotation plate along a splitting line extending along the radius ofthe optical disc with an optical axis as the center, a lightpolarization hologram arranged between the laser light source and theoptical rotation means, and light receiving means for receiving thereflected light from the optical disc incident via the light polarizinghologram and outputting a detection signal of an output levelcorresponding to the volume of the received light. The light polarizinghologram transmits an outgoing light from the laser light source as itis from the laser light source without bending its light path andradiates a reflected light of the outgoing light which is illuminated onthe optical disc along a light path bent responsive to the direction ofpolarization.

The light receiving means has at least three photodetectors forrespectively receiving a O-order diffracted light, a +one orderdiffracted light component and a -one order diffracted light componentof the reflected light from the optical disc passed through the opticalrotation means by the light polarization hologram.

The light receiving means has a photodetector for receiving the +oneorder diffracted light component and a photodetector for receiving the-one order diffracted light component. Each photodetector has aplurality of light receiving areas obtained by splitting into at leasttwo along a splitting line extending along the radius of the opticaldisc.

At least three photodetectors of the light receiving means are arrangedso that, when the photodetector of such at least three photodetectorsreceiving the O-order diffracted light component is in the just-focusstate, the volume of light received by one of such at least threephotodetectors receiving the +one order diffracted light component willbe equal to the volume of light received by one of such at least threephotodetectors receiving the -one order diffracted light component.

The light polarizing hologram is a planar hologram in the shape of adiffraction lattice having a uniform lattice spacing throughout anentire hologram region. The photodetector receiving the +one orderdiffracted light component and the photodetector receiving the -oneorder diffracted light component are mounted with a pre-set distanceahead and at back of the optical axis with respect to the photodetectorof such at least three photodetectors of the light receiving meansreceiving the O-order diffracted light component.

The light polarizing hologram is a substantially planar hologram in theshape of a diffractive lattice presenting a curvature so that a focalpoint with respect to the -one order diffracted light component differsfrom a focal point with respect to the -one order diffracted lightcomponent. The light receiving means has such at least threephotodetectors arranged on substantially the same plane.

The light polarizing hologram is split by a splitting line extendingalong the radius of the optical disc into an area having formed thereina light polarizing hologram in the shape of a diffractive lattice havinga coarse diffraction lattice spacing and an area having formed therein alight polarizing hologram in the shape of a diffractive lattice having adense diffraction lattice spacing. The light receiving means hasphotodetectors for receiving the +one order diffracted light componentand the -one order diffracted light component of the reflected lighthaving passed through the left rotation plate of said optical rotationmeans and having the light path bent by the area of the light polarizinghologram having one of the light polarizing patterns of the lightpolarizing hologram. The light receiving means also has photodetectorsfor receiving the +one order diffracted light component and the -oneorder diffracted light component of the reflected light having passedthrough the right rotation plate of the optical rotation means andhaving the light path bent by the area of the light polarizing hologramhaving the other light polarizing patterns of the light polarizinghologram.

The light polarization hologram is split into four areas each having acenter angle of 90° and having light polarizing hologram patterns formedfor bending the reflected light having passed through the opticalrotation means in four respectively different directions and radiatingthe thus bent reflected light. The light receiving means has aphotodetector for receiving the O-order diffracted light component bythe light polarizing hologram of the reflected light from the opticaldisc having passed through the optical rotation means and fourphotodetectors for receiving the reflected light bent in the fourdirections by the light polarizing hologram.

The light polarizing hologram, optical rotation means and the objectivelens are unitarily constructed as a movable unit servo-controlled on thebasis of a detection signal from the light receiving means. The laserlight source and the light receiving means are unitarily constructed asa laser module associated with the movable unit.

An optical disc driving unit according to the present invention includesa laser light source for radiating a laser light, an objective lens forradiating an outgoing light from the laser light source to an opticaldisc, optical rotation means arranged between the laser light source andthe objective lens and being split into a right rotation plate and aleft rotation plate along a splitting line extending along the radius ofthe optical disc with an optical axis as the center, a lightpolarization hologram arranged between the laser light source and theoptical rotation means, and light receiving means for receiving thereflected light from the optical disc incident via the light polarizinghologram and outputting a detection signal of an output levelcorresponding to the volume of the received light. The light polarizinghologram transmits an outgoing light from the laser light source as itis from the laser light source without bending its light path andradiating a reflected light of the outgoing light which is illuminatedon the optical disc along a light path bent responsive to the directionof polarization. The light polarizing hologram, optical rotation meansand the objective lens are unitarily constructed as a movable unitservo-controlled on the basis of a detection signal from the lightreceiving means. The laser light source and the light receiving meansare unitarily constructed as a laser module associated with the movableunit. The optical disc driving apparatus also includes disc rotatingdriving means for rotating the optical disc and servo control means forcontrolling the movable unit based upon a detection signal from thelight receiving means of the optical pickup device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of a conventional optical pickup devicefor reproducing data recorded on a magneto-optical disc.

FIG. 2 illustrates the structure of a conventional optical pickup devicefor reproducing data recorded on a phase-change optical disc.

FIG. 3 is a block diagram showing the structure of an optical discdriving apparatus employing an optical pickup device according to thepresent invention.

FIG. 4 illustrates a typical structure of am optical pickup device ofthe present invention employed in the above optical disc drivingapparatus.

FIG. 5 illustrates the state of separation of a reflected light by alight polarizing hologram provided on a movable unit of the opticalpickup device shown in FIG. 4.

FIG. 6 shows the positional relation of photodetectors provided on alaser module of the optical pickup device shown in FIG. 4 and the stateof radiation of the reflected light.

FIG. 7 illustrates the state of optical rotation of an outgoing lightfrom the laser module having passed through a right rotation plate of atwo-segment optical rotating plate provided on a movable unit of theoptical pickup device shown in FIG. 4.

FIG. 8 illustrates the state of optical rotation of an outgoing lightfrom the laser module having passed through a left rotation plate of thetwo-segment optical rotation plate.

FIG. 9 illustrates an illustrative structure of a signal detectioncircuit in the optical pickup device shown in FIG. 4.

FIG. 10 illustrates another typical structure of an optical pickupdevice of the present invention employed in the above optical discdriving apparatus.

FIG. 11 illustrates the state of separation of a reflected light by alight polarizing hologram provided on a movable unit of the opticalpickup device shown in FIG. 10.

FIG. 12 shows the positional relation of photodetectors provided on alaser module of the optical pickup device shown in FIG. 10 and the stateof radiation of the reflected light.

FIG. 13 illustrates yet another typical structure of an optical pickupdevice of the present invention employed in the above optical discdriving apparatus.

FIG. 14 illustrates the state of separation of the reflected light bythe polarizing hologram provided in a movable unit of the optical pickupdevice shown in FIG. 13.

FIG. 15 shows the positional relation of photodetectors provided on alaser module of the optical pickup device shown in FIG. 12 and the stateof radiation of the reflected light.

FIG. 16 illustrates a further typical structure of an optical pickupdevice of the present invention employed in the above optical discdriving apparatus.

FIG. 17 shows the positional relation of photodetectors provided on alaser module of the optical pickup device shown in FIG. 16 and the stateof radiation of the reflected light.

FIG. 18 illustrates the structure of a movable unit in the furtherstructure of the optical pickup device of the present invention employedin the optical disc driving apparatus.

FIG. 19 illustrates the positional relation of a photodetector providedon a laser module constituting the optical disc driving apparatus of thepresent invention along with the movable unit shown in FIG. 18 and thestate of radiation of the reflected light.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the drawings, the best mode for carrying out the presentinvention will be explained in detail.

The optical pickup device according to the present invention is employedas an optical head device in an optical disc driving apparatusconfigured as shown for example in FIG. 3.

In the optical disc driving apparatus, an optical head device 31records/reproduces signals by optically scanning the recording surfaceof an optical disc 33 rotationally driven by a spindle motor 32 forrecording/reproducing signals, and is configured for being movedradially of the disc by a linear motor 34. With the optical head device31, the amount of the outgoing laser light beam is controlled by a laserpower control circuit 35 so that the recording surface of the opticaldisc 33 is scanned with a pre-set volume of the laser beam suitable forrecording/reproduction. The optical head device 31 transmits a trackingerror signal THE and a focusing error signal FE, obtained on opticallyscanning the recording surface of the optical disc 33, so that servocontrol such as tracking or focusing control is done on the basis of thetracking error signal THE and the focusing error signal FE by a servocontrol circuit 36.

The present optical disc driving apparatus is adapted for employing aplay-only optical disc or a variety of recordable optical discs, such asa phase-change optical disc or a magneto-optical disc. The optical discdriving apparatus is configured for being set by a system controller 37to recording/reproducing modes for coping with the various opticaldiscs.

For the recording mode for the phase-change optical disc, recordingsignals are processed by a signal processing circuit 38 with pre-setencoding or appendage of error correction codes. The processed recordingsignals are supplied to the laser power control circuit 35. The laserpower control circuit 35 controls the light volume of the laser lightbeam radiated by the optical head device 31 in accordance with therecording signals for light modulating the laser beam with the recordingsignals. The optical head device 31, servo-controlled as describedabove, scans a desired area of the recording surface of the optical disc31 with the laser beam light-modulated with the recording signals forrecording signals on a phase-change recording area of the phase-changeoptical disc.

For the recording mode for the magneto-optical disc, recording signalsare processed by the signal processing circuit 38 with pre-set encodingor appendage of error correction codes. The processed recording signalsare supplied to a magnetic head driving circuit 39. The magnetic headdriving circuit 39 drives a magnetic head 40 responsive to the recordingsignals. The magnetic head 40, facing the optical head device 31 withthe optical disc 33 in-between, is moved by the linear motor 34 alongthe radius of the disc along with the optical head device 31. As theservo control as described above is going on, a desired area on therecording surface of the optical disc 33 is scanned by a laser beam fromthe optical head device 31, at the same time as it is scanned with amodulated magnetic field corresponding to the recording signals. Thusthe modulated magnetic field, corresponding to the recording signals, isimpressed on a magneto-optical area (MO area) raised in temperature tothe Curie temperature by irradiation with the laser beam for recordingsignals on the MO area.

For the playback mode for the play-only optical disc or the phase-changeoptical disc, a desired area on the recording surface of the opticaldisc is optically scanned by the optical head device, as servo controlas described above is going on, so that playback RF signals are producedfrom the optical disc or the phase-change optical disc and supplied tothe signal processing circuit 38. The signal processing circuit 38processes the playback RF signals supplied from the optical head device31 with pre-set demodulation or error correction for generating playbacksignals.

For the playback mode for the magneto-optical disc, a desired area onthe recording surface of the magneto-optical disc is optically scannedby the optical head device, as servo control as described above is goingon, so that playback RF signals are produced from the magneto-opticaldisc is supplied to the signal processing circuit 38. The signalprocessing circuit 38 processes the playback RF signals supplied fromthe optical head device 31 with pre-set demodulation or error correctionfor generating playback signals. The optical pickup device, employed asthe above-mentioned optical head device 31, is made up of a laser module41 and a movable unit 50, as shown for example in FIGS. 4 and 5.

The laser module 41 is constructed as shown and described in JP PatentKokai Publication JP-A-7-114746 proposed by the present Assignee, and iscomprised of a laser diode 42 for radiating a laser beam in a directionparallel to the magneto-optical disc 33, a reflecting mirror 43 forreflecting the outgoing light from the laser diode 42 towards themagneto-optical disc 33 and first to fifth photodetectors 44 to 48.

These photodetectors 44 to 48 are arranged side-by-side along the radiusof the disc for receiving five reflected light portions split by a lightpolarizing hologram 51 in the movable unit 50, as shown in FIG. 6.

The movable unit 50 includes the polarizing hologram 51, functioning asa polarizing beam splitter, a two-segment optical rotation plates 52divided by a partitioning line extending along the radius of the discpassing through the optical axis into a right rotation plate 52R and aleft rotation plate 52L, and an objective lens 53. The polarizinghologram 51 and the two-segment optical rotation plate 52 are of a discshape of the same diameter and bonded together to form a unitarystructure. The objective lens 53 is of the same diameter as thepolarizing hologram 51 and the two-segment optical rotation plate 52 andare layered along with the polarizing hologram 51 and the two-segmentoptical rotation plate 52 within a cylindrically-shaped holder 54.

The polarizing hologram 51 operates as a polarizing beam splitter fordirectly transmitting the incident light of the P-polarized componentstherethrough and for radiating the incident light of the S-polarizedcomponent having the direction of light polarization thereofperpendicular to the P-polarized component along a light path differentfrom the light path of the incident light. The polarizing hologram 51 isdivided into two along a partition line extending along the radius ofthe disc through the optical axis. The optical hologram 51 has an area51A in register with the right rotation plate 52R and an area 51B inregister with the left rotation plate 52R. In these areas 51A and 51Bare formed a planar hologram in the form of a diffractive lattice havinga coarser lattice spacing and a planar hologram in the form of adiffractive lattice having a denser lattice spacing, respectively.

The right rotation plate 52R rotates the incident light a pre-set angleα, herein 22.5°, towards right as seen from the outgoing side, andradiates the thus rotated light. The left rotation plate 52R rotates theincident light a pre-set angle α, herein 22.5°, towards left as seenfrom the outgoing side, and radiates the thus rotated light.

The movable unit 50 is driven under control by the servo control circuit36 on the basis of the tracking error signal THE and a focusing errorsignal FE.

The movable unit 50 radiates the outgoing light of the laser module 41,incident thereon via the polarizing hologram 51 and the two-segmentoptical rotation plate 52, on the optical disc 33 as a converging lightbeam by the objective lens 53. The reflected light by the optical disc33 is split from the objective lens 53 via the two-segment opticalrotation plate 52 and the polarizing hologram 51 into light portionsproceeding along five light paths. These light portions fall on thelaser module 41.

The first photodetector 44 of the laser module 41 is provided on theback side of the reflective mirror 43, as shown in FIG. 6. The firstphotodetector 44 is configured for receiving, by a light receivingportion SE, about one-half of the O-order light component of thereflected light which is illuminated thereon without being changed inits light path by the light polarizing hologram 51.

Each of the second to fourth photodetectors 45 to 48 is a two-segmentphotodetector divided along a partition line extending along the radiusof the disc with an interval of, for example, 1 to 2 μm.

The second photodetector 45 receives the +one order diffracted lightcomponent from the left rotation plate 52L split by the polarizinghologram 51 by light receiving areas SA and SB. The third photodetector46 receives the +one order diffracted light component of the reflectedlight from the left rotation plate 52R split by the polarizing hologram51 by light receiving areas SC and SD. The fourth photodetector 47receives the -one order diffracted light component of the reflectedlight from the left rotation plate 52R split by the polarizing hologram51 by light receiving areas SF and SG. The fifth photodetector 48receives the -one order diffracted light component from the leftrotation plate 52L split by the polarizing hologram 51 by lightreceiving areas SH and SI.

In the above-described optical pickup device, the laser module 41radiates a laser beam of the P-polarized light component from the laserdiode 42 as an outgoing light in a direction parallel to the opticaldisc 33. This outgoing light is reflected by the reflective mirror 43,having an angle of reflection of 45°, in a direction perpendicular tothe magneto-optical disc 7, so as to fall on the movable unit 50.

The outgoing light of the laser module 41 is incident on the two-segmentoptical rotation plate 52 via the light polarizing hologram 51configured for directly transmitting the P-polarized light component. Inthe outgoing light of the laser module 41, the light componentstransmitted through the right rotation plate 52R of the two-segmentoptical rotation plate 52 are rotated 22.5° towards right as seen fromthe objective lens 53, as shown in FIG. 7, while the light componentstransmitted through the left rotation plate 52L of the two-segmentoptical rotation plate 52 are rotated 22.5° towards left as seen fromthe objective lens 53, as shown in FIG. 8.

These light components of the outgoing light, thus rotated by thetwo-segment optical rotation plate 52, are condensed by the objectivelens 53 so as to be illuminated on the magneto-optical disc 33.

The outgoing light radiated on the magneto-optical disc 33 by theobjective lens 53 is reflected by the magneto-optical disc 33. Theoutgoing light components, rotated towards right and towards left asdescribed above, are reflected as they are rotated by an angle θktowards right and towards left, respectively, by the Kerr effect, inaccordance with data recorded on the magneto-optical disc 33.

This reflected light then falls via the objective lens 53 on thetwo-segment optical rotation plate 52. The components transmittedthrough the right rotation plate 52R in the state of the outgoing lightare incident on the left rotation plate 52L in the state of thereflected light, while the components transmitted through the leftrotation plate 52L in the state of the outgoing light are incident onthe right rotation plate 52R in the state of the reflected light. Thereflected light incident on the left rotation plate 52L is radiated asit is thereby rotated further by 22.5° towards left as seen from thelaser module 41, while the reflected light incident on the rightrotation plate 52R is radiated as it is thereby rotated further by 22.5°towards right as seen from the laser module 41.

Thus the reflected light outgoing from the left rotation plate 52L isradiated as it is rotated -45°+θk (-22.5°-22.5°+θk (datacomponent)=-45°+θk) towards left, as viewed from the laser module 41,with respect to the outgoing light incident on the right rotation plate52R. On the other hand, the reflected light outgoing from the rightrotation plate 52R is radiated as it is rotated 45°+θk (22.5°+22.5°+θk(data component)=45°+θk) towards left, as viewed from the laser module41, with respect to the outgoing light incident on the left rotationplate 52L.

The reflected light, rotated in different directions by the two-segmentoptical rotation plate 52, falls on the light polarizing hologram 51.

Since the reflected light has been rotated ±45°+θk towards right or leftas seen from the laser module 41, it has both P-polarized lightcomponents and S-polarized light component. It is noted that the lightpolarizing hologram 51, radiating the S-polarized light components afterbending its light path a pre-set angle, as described above, has suchcharacteristics that the light of the S-polarized light componentsrotated towards right as seen from the laser module 41 is radiated alonga light path bent towards left, while the light of the same S-polarizedlight components rotated towards left as seen from the laser module 41is radiated along a light path bent towards right.

The reflected light transmitted through the left rotation plate 52L hasthe S-polarized component, rotated towards right as seen from the lasermodule 41. Thus the reflected light has its light path bent a pre-setangle towards left by the area 51B of the light polarizing hologram 51with dense diffraction lattice, so that its +one order diffracted lightcomponent and -one order diffracted light component are radiated on thesecond photodetector 45 and the fifth photodetector 48 of the lasermodule 41, respectively. On the other hand, the reflected lighttransmitted through the right rotation plate 52R has the S-polarizedcomponent, rotated towards left as seen from the laser module 41. Thusthe reflected light has its light path bent a pre-set angle towardsright by the area 51A of the light polarizing hologram 51 with coarsediffraction lattice, so that its +one order diffracted light componentand -one order diffracted light component thereof are radiated on thethird photodetector 46 and the fourth photodetector 47 of the lasermodule 41, respectively.

The reflected light of the P-polarized light components, transmittedthrough the light polarizing hologram 20, is radiated on the firstphotodetector 44 with its light path remaining unchanged.

Under the just-focus state, the photodetectors 44 to 48 receive thereflected light portions as points produced by beam spots representingthe limit of diffraction. However, in the far focus defocused state, inwhich the objective lens 53 approaches to the optical disc 33, the lightreceiving area SB of the second photodetector 45, the light receivingarea SC of the third photodetector 46, the light receiving areas SF ofthe fourth photodetector 47 and the light receiving areas SI of thefifth photodetector 48 receive crescent-shaped reflected light portions,as shown in FIG. 6. In the near-focus defocused state in which theobjective lens 53 is moved away from the optical disc 33, it is thelight receiving area SA of the second photodetector 45, the lightreceiving area SD of the third photodetector 46, the light receivingareas SG of the fourth photodetector 47 and the light receiving areas SHof the fifth photodetector 48 that receive crescent-shaped reflectedlight portions.

The photodetectors 44 to 48 supply detection signals A to I of signallevels corresponding to the light volume of the reflected light receivedby the light receiving areas SA to SI to a signal detection circuitconfigured as shown in FIG. 9.

The signal detection circuit has eight adders 60 to 67 and twosubtractors 68 and 69.

The adder 60 sums detection signals A and H obtained by the lightreceiving areas SA and SH of the second and fifth photodetectors 44, 48,respectively. The sum signal A+H by the adder 60 is supplied to theadders 64 and 66.

The adder 61 sums detection signals B and I obtained by the lightreceiving areas SB and SI of the second and fifth photodetectors 44, 48,respectively. The sum signal B+I by the adder 61 is supplied to theadders 64 and 66.

The adder 62 sums detection signals C and F obtained by the lightreceiving areas SC and SF of the third and fourth photodetectors 45, 47,respectively. The sum signal C+F by the adder 62 is supplied to theadders 65 and 67.

The adder 63 sums detection signals D and G obtained by the lightreceiving areas SD and SG of the third and fourth photodetectors 45, 47,respectively. The sum signal D+G by the adder 63 is supplied to theadders 65 and 67.

The adder 64 sums the sum signal A+H by the adder 60 to the sum signalB+I by the adder 61. The resulting sum signal A+H+B+I by the adder 64 issupplied to the subtractor 68.

The adder 65 sums the sum signal C+F by the adder 62 to the sum signalD+G by the adder 63. The resulting sum signal C+F+D+G by the adder 65 issupplied to the subtractor 68.

The adder 66 sums the sum signal A+H by the adder 60 to the sum signalD+G by the adder 63. The resulting sum signal A+H+D+G by the adder 66 issupplied to the subtractor 69.

The adder 67 sums the sum signal B+I by the adder 61 to the sum signalC+F by the adder 62. The resulting sum signal B+I+C+F by the adder 67 issupplied to the subtractor 69.

The subtractor 68 subtracts the sum signal C+F+D+G from the sum signalA+H+B+I by the adder 64. The subtraction signal (A+H+B+I)-(C+F+D+G)represents a playback MO signal.

The subtractor 69 subtracts the sum signal B+I+C+F from the sum signalA+H+D+G by the adder 68. The subtraction signal (A+H+D+G)-(B+I+C+F)represents the focusing error signal FE.

The signal detection circuit outputs a detection signal E obtained fromthe light receiving area SE of the first photodetector 44 directly asthe playback RF signal.

That is, the signal detection circuit generates

    FE=(A+H+D+G)-(B+I+C+F)

    MO=(A+H+B+I)-(G+D+F+C)

    RF=E

based upon the detection signals A to I of signal levels correspondingto the light volumes of the reflected light received by the lightreceiving areas SA to SI of the photodetectors 44 to 48.

The servo control circuit 36 of the recording/reproducing apparatuseffectuates focusing control by driving the movable unit 50 in adirection of correcting the defocusing based upon the focusing errorsignals FE supplied from the signal detection circuit. The playback MOsignals or playback RF signals are decoded by the signal processingcircuit 38 to produce playback signals which are transmitted to anexternally connected computer or speaker.

The optical pickup device according to the present invention iscomprised of a laser module 141 and a movable unit 150, as shown forexample in FIGS. 10 and 11.

The present optical pickup device differs from the optical pickup deviceshown in FIGS. 3 and 4 as to the polarizing hologram 151 in the movableunit 150 and as to the first to third photodetectors 144 to 146 in thelaser module 141, while the remaining components are the same as thoseof the optical pickup device shown in FIGS. 3 and 4. Therefore, thecomponents of the present optical pickup device are denoted by the samereference numerals and the corresponding description is omitted forclarity.

With the present optical pickup device, a planar hologram in the form ofa diffractive lattice having a uniform lattice interval is formed on theentire area of the light polarizing hologram 151 in the movable unit150, for splitting the reflected light passed through the two-segmentoptical rotation plate 52 into three light portions, namely a O-orderdiffracted light component and ±one order diffracted light components.

The first to third photodetectors 144 to 146 in the laser module 141 areprovided in different heights, so that, when the first photodetector 144receives the O-order diffracted light component of the reflected lightfrom the movable unit 150 under the just-focus state, the secondphotodetector 145 receives the +one order diffracted light componentunder the near-focus state, while the third photodetector 146 receivesthe -one order diffracted light component under the far-focus state,with the second and third photodetectors 145, 146 receiving the sameamount of light.

The first photodetector 144 in the laser module 141 is designed as atwo-segment photodetector, with a partitioning line extending in adirection perpendicular to the radial direction of the disc, as shown inFIG. 12. The first photodetector 144 is configured for receiving aboutone-half of the O-order light component, radiated by the polarizinghologram 151 without being changed as to light path, by light receivingareas SE and SF.

Each of the second and third photodetectors 145, 146 is a four-segmentphotodetector, with partitioning lines extending along the radialdirection of the disc.

The second photodetector 145 receives +one order diffracted lightcomponent of the reflected light, split by the light polarizing hologram151, by light receiving areas SA, SB, SC and SD. The third photodetector146 receives -one order diffracted light component of the reflectedlight, split by the light polarizing hologram 151, by light receivingareas SG, SH, SI and SJ.

With the above-described optical pickup device, the following arithmeticoperations are executed by a signal detection circuit comprised ofadders and subtractors on the basis of detection signals A to J ofsignal levels corresponding to light volume of the reflected lightreceived by light receiving areas SA to SJ of the photodetectors 144 to146 for producing playback MO signals, focusing error signals FE,playback RF signals and push-pull signals PP.

That is, the playback MO signals and the focusing error signals FE maybe produced, from the detection signals A to D and G to J by thephotodetectors SA to SD and SG to SJ of the second and thirdphotodetectors 145 and 146, respectively receiving the +one orderdiffracted light component and -one order diffracted light component, inaccordance with an arithmetic operation:

    MO=(A+B+I+J)=(C+D+G+H)

and in accordance with an arithmetic operation:

    FE=(A+D+H+I)=(B+C+G+J)

respectively.

The playback RF signals and the tracking error signals THE may beproduced from detection signals E and F of the light receiving sectionsSE and SF of the first photodetector 144 receiving the O-orderdiffracted light component of the reflected light in accordance with anarithmetic operation:

    RF=E+F

and in accordance with an arithmetic operation:

    THE=E-F

The playback RF signals may also be produced from the detection signalsA to J by the light receiving areas SA to SJ of the photodetectors 144to 146 in accordance with an arithmetic operation:

    RF=A+B+C+D+E+F+G+H+I+J

The optical pickup device according to the present invention iscomprised of a laser module 241 and a movable unit 250, as shown forexample in FIGS. 13 and 14.

The present optical pickup device differs from the optical pickup deviceshown in FIGS. 11 and 12 as to the polarizing hologram 251 in themovable unit 250 and as to the first to third photodetectors 244 to 246in the laser module 241, while the remaining components are the same asthose of the optical pickup device shown in FIGS. 11 and 12. Therefore,the components of the present optical pickup device are denoted by thesame reference numerals and the corresponding description is omitted forclarity.

That is, the light polarizing hologram 251 in the movable unit 250 ofthe present optical pickup device, configured for splitting thereflected light passed through the two-segment optical rotation plate52, into the O-order diffracted light component and ±one orderdiffracted light component, has a focal point for the +one orderdiffracted light component different from a focal point for the -oneorder diffracted light component. It is noted that the first to thirdphotodetectors 244 to 246 in the laser module 241 are arranged on oneand the same plane. However, an arrangements so made that, when thefirst photodetector 244 receives the O-order diffracted light componentof the reflected light from the movable unit 250 under the just-focusstate, the second photodetector 245 receives the +one order diffractedlight component under the near-focus state and the third photodetector246 receives the -one order diffracted light component under thefar-focus state, with the second and the third photodetectors 245, 246receiving the same amount of light.

The light polarizing hologram 251 forms a substantially planar hologramin the shape of a diffraction lattice having a curvature, for providinga focal point for the +one order diffracted light component and a focalpoint for the -one order diffracted light component different from eachother, as shown schematically in FIG. 15.

The first photodetector 244 in the laser module 241 is a two-segmentphotodetector with a partitioning line extending along a directionperpendicular to the radial direction of the disc. The firstphotodetector 244 is designed to receive about one-half of the O-orderlight component, radiated thereto without having its light path changedby the light polarizing hologram 251, by its respective light receivingareas.

Each of the second and third photodetectors 245, 246 is a four-segmentphotodetector, with respective partitioning lines extending along theradial direction of the disc.

The second photodetector 245 receives +one order diffracted lightcomponents of the reflected light split by the light polarizing hologram251 by respective light receiving areas. The third photodetector 245receives -one order diffracted light components of the reflected lightsplit by the light polarizing hologram 251 by respective light receivingareas.

With the above-described optical pickup device, the signal detectioncircuit executes arithmetic operations similar to those executed by theoptical pickup device shown in FIGS. 11 and 12, based upon detectionsignals A to J of signal levels corresponding to the light volume of thereflected light received by the light receiving areas of thephotodetectors 244 to 246, for producing playback MO signals, focusingerror signals FE, playback MO signals and the tracking error signalsTHE.

That is, the playback MO signals and the focusing error signals may beproduced, from the detection signals A to D and G to J by the respectivelight receiving sections of the second and third photodetectors 245, 246receiving the +one order diffractive light components and -one orderdiffracted light components of the reflected light, in accordance withthe following arithmetic operations:

    MO=(A+B+I+J)-(C+D+G+H)

and

    FE=(A+D+H+I)-(B+C+G+H)

respectively.

On the other hand, the playback RF signals and the tracking errorsignals THE may be produced, from detection signals E and F of therespective light receiving sections of the first photodetector 244receiving the O-order diffracted light component of the reflected light,in accordance with the arithmetic operations:

    RF=E+F

    THE=E-F

respectively.

The optical pickup device according to the present invention iscomprised of a laser module 341 and a movable unit 350, as shown forexample in FIGS. 16 and 17.

The present optical pickup device differs from the optical pickup deviceshown in FIGS. 14 and 15 as to the polarizing hologram 351 in themovable unit 350 and as to the first to third photodetectors 344 to 346in the laser module 341. However, since the remaining components are thesame as those of the optical pickup device shown in FIGS. 14 and 15, thecomponents of the present optical pickup device are denoted by the samereference numerals and the corresponding description is omitted forclarity.

That is, the light polarizing hologram 351 in the movable unit 350 ofthe present optical pickup device, configured for splitting thereflected light passed through the two-segment optical rotation plate52, into the O-order diffracted light component and ±one orderdiffracted light component, has a focal point for the +one orderdiffracted light component different from a focal point for the -oneorder diffracted light component. It is noted that the first to thirdphotodetectors 344 to 346 in the laser module 341 are arranged on oneand the same plane. However, the arrangement is so made that, when thefirst photodetector 344 receives the O-order diffracted light componentof the reflected light from the movable unit 350 under the just-focusstate, the second photodetector 345 receives the -one order diffractedlight component under the near-focus state and the third photodetector346 receives the +one order diffracted light component under thefar-focus state, with the second and the third photodetectors 345, 346receiving the same amount of light.

The first photodetector has only a light receiving area SI and isconfigured for receiving the reflected light of the P-polarized lightcomponent. The second photodetector 345 has its light receiving areasplit into light receiving areas SA to SD along the radius of the discfor receiving the -one order diffracted light component of the reflectedlight of the S-polarized light component having passed through the leftrotation plate 52L and having its light path bent by the lightpolarizing hologram 351. The third photodetector 346 has its lightreceiving area split into light receiving areas E to H along the radiusof the disc for receiving the +one order diffracted light component ofthe reflected light of the S-polarized light component having passedthrough the right rotation plate 52R and having its light path bent bythe light polarizing hologram 351.

With the optical pickup device, having above-described laser module 341,the focusing error signal, playback MO signal, tracking error signal THEand the playback RF signal may be produced, from the detection signals Ato I of signal levels corresponding to the light volumes of thereflected light received by the light receiving areas SA to SI of thephotodetectors 344 to 346 of the laser module 341, by executing thefollowing arithmetic operations:

    FE=(A+D+F+G)-(B+C+E+F)

    MO=(A+B+C+D)-(E+F+G+H)

    THE=(A+B+C+D)-(C+D+E+F)

    RF=I

by the signal detection circuit comprised of adders and subtractors.

The optical pickup device according to the present invention iscomprised of a movable unit 450, as shown for example in FIG. 18, and alaser module 441 as shown for example in FIG. 19.

The present optical pickup device differs from the optical pickup deviceshown in FIGS. 3 and 4 as to the polarizing hologram 451 in the movableunit 450 and as to the first to third photodetectors 444 to 448 in thelaser module 441, while the remaining components are the same as thoseof the optical pickup device shown in FIGS. 14 and 15. Therefore, thecomponents of the present optical pickup device are denoted by the samereference numerals and the corresponding description is omitted forclarity.

That is, in the present optical pickup device, the light polarizinghologram 451 in the movable unit 450, is divided into four areas, with acenter angle each of 90°, in which four light polarizing hologrampatterns, that is a first light polarizing hologram pattern 451a to afourth light polarizing hologram pattern 451d, are formed, as shown inFIG. 18a.

The light polarizing hologram patterns, formed in the light polarizinghologram 451, are formed in adjacency to one another. Specifically, thefirst light polarizing hologram pattern 451a has characteristics ofbending the light path of the reflected light of the S-polarized lightcomponent from the right rotation plate 52R of the two-segment opticalrotation plate 52 towards left as seen from the laser module 441. Thesecond light polarizing hologram pattern 451b has characteristics ofbending the light path of the reflected light of the S-polarized lightcomponent from the right rotation plate 52R of the two-segment opticalrotation plate 52 towards right as seen from the laser module 441. Thethird light polarizing hologram pattern 451c has characteristics ofbending the light path of the reflected light of the S-polarized lightcomponent from the left rotation plate 52L of the two-segment opticalrotation plate 52 towards left as seen from the laser module 441. Inaddition, the fourth light polarizing hologram pattern 451d hascharacteristics of bending the light path of the reflected light of theS-polarized light component from the left rotation plate 52L of thetwo-segment optical rotation plate 52 towards right as seen from thelaser module 441.

On the other hand, the laser module 441 is provided with second to fifthphotodetectors 445 to 448, centered about a first photodetector 444configured for receiving the reflected light of the P-polarized lightcomponent, radiated along a light path unaffected by the lightpolarizing hologram 451, as shown in FIG. 19a.

The second to fifth photodetectors 445 to 448 have light receiving areasthereof split by splitting lines extending along the radius of the discinto respective two light receiving areas or portions, that is intolight receiving areas SA and SB, light receiving areas SB and SC, lightreceiving areas light SC and SD and light receiving areas SG and SH,respectively. The first photodetector 444 has a light receiving area SIwhich is about one-half of the light receiving area of each of thephotodetectors 445 to 448.

The first photodetector 445 is provided at a position of receiving thereflected light having its light path bent by the fourth lightpolarizing hologram pattern 451d of the light polarizing hologram 451.The second photodetector 446 is provided at a position of receiving thereflected light having its light path bent by the second lightpolarizing hologram pattern 451b. The third photodetector 447 isprovided at a position of receiving the reflected light having its lightpath bent by the third light polarizing hologram pattern 451c. Thefourth photodetector 448 is provided at a position of receiving thereflected light having its light path bent by the third light polarizinghologram pattern 451a.

With the above-described construction of the optical pickup device, theS-polarized light component of the reflected light, rotated towards leftby the left rotation plate 52L of the two-segment rotation plate 52 asdescribed above, is incident on the third and fourth light polarizinghologram patterns 451c, 451d of the light polarizing hologram 451, whilethe S-polarized light component of the reflected light, rotated towardsright by the right rotation plate 52R of the two-segment rotation plate52 as described above, is incident on the first and second lightpolarizing hologram patterns 451a, 451b of the light polarizing hologram451.

The third light polarizing hologram pattern 451c bends the light path ofthe reflected light towards left as seen from the laser module 441 andradiates the light on the fourth photodetector 447. The fourth lightpolarizing hologram pattern 451d bends the light path of the reflectedlight towards right and radiates the light on the second photodetector445. The first light polarizing hologram pattern 451a bends the lightpath of the reflected light towards left and radiates the light on thefifth photodetector 448. The second light polarizing hologram pattern451b bends the light path of the reflected light towards right andradiates the light on the third photodetector 446.

The reflected light of the P-polarized light component, having passedthrough the two-segment rotation plate 52, is radiated on the firstphotodetector 444 without having its light path changed by the lightpolarizing hologram.

Specifically, the reflected light is radiated under the just-focus stateas a point on each of the second to fifth photodetectors 445 to 448 asshown in FIG. 19a. However, under the near-focus defaces state in whichthe objective lens 53 is moved away from the optical disc 33, thereflected light is radiated in a sector shape in each of the lightreceiving area SA of the second photodetector 445, light receiving areaSF of the third photodetector 446, the light receiving area SC of thefourth photodetector 447 and the light receiving area SH of the fifthphotodetector 448, as shown in FIG. 19b. Under the far-focus defocusedstate in which the objective lens 53 is moved towards the optical disc33, the reflected light is radiated in a sector shape in each of thelight receiving area SB of the second photodetector 445, light receivingarea SE of the third photodetector 446, light receiving area SD of thefourth photodetector 447 and the light receiving area SG of the fifthphotodetector 448, as shown in FIG. 19c.

Meanwhile, the first photodetector 444 is irradiated with the reflectedlight of the P-polarized light component both under the near-focus andfar-focus states over a wider area than under the just-focus state.

With the optical pickup device having the above-described constructionof the laser module 441, the focusing error signal FE, playback MOsignal, tracking error signal THE and the playback RG signal may beproduced by executing, on the basis of the detection signals A to I ofsignal levels corresponding to the light volumes of the reflected lightreceived by the light receiving areas AS to SI of the photodetectors 444to 448, the following arithmetic operations:

    FE=(B+D+E+G)-(A+C+F+H)

    MO=(A+B+C+D)-(E+F+G+H)

    THE=(A+B+E+F)-(C+D+G+H)

    RF=I

by the signal detection circuit made up of adders and subtractors.

In the above-described embodiments, the right rotation plate 52R and theleft rotation plate 52L of the two-segment light rotation plate 52 aredeigned to rotate the outgoing light and the reflected light by 22.5°,respectively. However, this is merely illustrative and a variety ofmodifications inclusive of those of numerical figures may be made withinthe scope of the invention.

With the optical pickup device according to the present invention, it isunnecessary to provide a polarizing beam splitter or a beam splitter inthe light path for extracting the reflected light so that the number ofcomponent parts may be diminished. Thus the overall optical path may bereduced and the construction simplified for reducing the production costof the optical pickup device and an optical disc driving apparatus, suchas a magneto-optical disc reproducing apparatus, employing the opticalpickup device.

On the other hand, there is a relation of trade-off between the couplingefficiency and the S/N ratio, such that, if the coupling efficiency isto be improved, the S/N ratio is lowered and, conversely, if the S/Nratio is to be improved, the coupling efficiency is lowered. However,since the light polarizing hologram operates only in the stage of thereflected light, without operating on the outgoing light, a highcoupling efficiency may be achieved. In addition, the three reflectedlight portions from the light polarizing hologram may directly bereceived substantially in their entirety by the photodetectors, so thata high S/N ratio may be achieved.

Since it suffices to control the position of the laser module forappropriately receiving the reflected light from the movable unit by therespective photodetectors, the optical pickup device may be easilyassembled or adjusted in its mounting position.

Since the movable unit in its entirety is driven responsive to thefocusing error signal or tracking error signals, the focusing errors andthe tracking errors may be corrected while the movable unit is kept inits initial pre-set position.

Likewise, the magneto-optical (MO) signals may be detected usingsubstantially 100% of the reflected light rotated towards right or leftresponsive to data recorded on the MO disc, the S/N ratio may beimproved, while wavelength variations may be coped with.

In addition, the in-phase noise components may be eliminated and onlysignal components may be amplified and detected by detecting the MOsignal by subtracting the light volume of the received light of theright-rotated S-polarized light component radiated on the firstphotodetector via the left rotation plate of the two-segment opticalrotation plate, so that the MO signals can be detected reliably.

Furthermore, since the playback RF signals can be generated fromdirectly received reflected light from the MO disc, signal componentsranging from low to high ranges are comprised in the RF signals. Thusthe playback RF signals with high S/N ratio may be formed, so thatchannel clocks may be correctly formed even in an optical disc employingthe sample-servo format.

I claim:
 1. An optical pickup device comprising:a laser light source forradiating a laser light; an objective lens for radiating an outgoinglight from the laser light source to an optical disc; optical rotationmeans arranged between said laser light source and said objective lensand being split into a right rotation plate and a left rotation platealong a splitting line extending along the radius of said optical discwith an optical axis as the center; a light polarizing hologram arrangedbetween said laser light source and said optical rotation means, saidlight polarizing hologram transmitting an outgoing light from said laserlight source as it is from said laser light source without bending itslight path and radiating a reflected light of said outgoing lightradiated on said optical disc along a light path bent responsive to thedirection of polarization; light receiving means for receiving thereflected light from said optical disc incident via said lightpolarizing hologram and outputting a detection signal of an output levelcorresponding to the volume of the received light; wherein said lightreceiving means has at least three photodetectors for respectivelyreceiving a O-order diffracted light, a +one order diffracted lightcomponent and a -one order diffracted light component of the reflectedlight from said optical disc passed through said optical rotation meansby said light polarizing hologram; wherein said at least threephotodetectors of said light receiving means are arranged so that, whenthe photodetector of said at least three photodetectors receiving theO-order diffracted light component is in the just-focus state, thevolume of light received by one of said at least three photodetectorsreceiving the +one order diffracted light component will be equal to thevolume of light received by one of said at least three photodetectorsreceiving the -one order diffracted light component; and wherein saidlight polarizing hologram is a planar hologram in the shape of adiffraction lattice having a uniform lattice spacing throughout anentire hologram region; and wherein, with respect to the photodetectorof said at least three photodetectors of said light receiving meansreceiving the O-order diffracted light component, the photodetectorreceiving the +one order diffracted light component and thephotodetector receiving the -one order diffracted light component aremounted with a pre-set distance ahead and at back of the optical axis.2. An optical pickup device comprising:a laser light source forradiating a laser light; an objective lens for radiating an outgoinglight from the laser light source to an optical disc; optical rotationmeans arranged between said laser light source and said objective lensand being split into a right rotation plate and a left rotation platealong a splitting line extending along the radius of said optical discwith an optical axis as the center; a light polarizing hologram arrangedbetween said laser light source and said optical rotation means, saidlight polarizing hologram transmitting an outgoing light from said laserlight source as it is from said laser light source without bending itslight path and radiating a reflected light of said outgoing lightradiated on said optical disc along a light path bent responsive to thedirection of polarization; light receiving means for receiving thereflected light from said optical disc incident via said lightpolarizing hologram and outputting a detection signal of an output levelcorresponding to the volume of the received light; and wherein saidlight polarizing hologram is split by a splitting line extending alongthe radius of the optical disc into an area having formed therein alight polarizing hologram in the shape of a diffractive lattice havingcoarse diffraction lattice spacing and an area having formed therein alight polarizing hologram in the shape of a diffractive lattice havingdense diffraction lattice spacing; said light receiving means havingphotodetectors for receiving the +one order diffracted light componentand the -one order diffracted light component of the reflected lighthaving passed through the left rotation plate of said optical rotationmeans and having the light path bent by the area of said lightpolarizing hologram having one of the light polarizing patterns of saidlight polarizing hologram, said light receiving means also havingphotodetectors for receiving the +one order diffracted light componentand the -one order diffracted light component of the reflected lighthaving passed through the right rotation plate of said optical rotationmeans and having the light path bent by the area of said lightpolarizing hologram having the other light polarizing patterns of saidlight polarizing hologram.
 3. An optical pickup device comprising:alaser light source for radiating a laser light; an objective lens forradiating an outgoing light from the laser light source to an opticaldisc; optical rotation means arranged between said laser light sourceand said objective lens and being split into a right rotation plate anda left rotation plate along a splitting line extending along the radiusof said optical disc with an optical axis as the center; a lightpolarizing hologram arranged between said laser light source and saidoptical rotation means, said light polarizing hologram transmitting anoutgoing light from said laser light source as it is from said laserlight source without bending its light path and radiating a reflectedlight of said outgoing light radiated on said optical disc along a lightpath bent responsive to the direction of polarization; light receivingmeans for receiving the reflected light from said optical disc incidentvia said light polarizing hologram and outputting a detection signal ofan output level corresponding to the volume of the received light; andwherein said light polarizing hologram is split into four areas eachhaving a center angle of 90° and having light polarizing hologrampatterns formed for bending the reflected light having passed throughthe optical rotation means in four respectively different directions andradiating the thus bent reflected light; said light receiving meanshaving a photodetector for receiving the O-order diffracted lightcomponent by the light polarizing hologram of the reflected light fromthe optical disc having passed through said optical rotation means andfour photodetectors for receiving the reflected light bent in the fourdirections by said light polarizing hologram.